Detector configurations for optical metrology

ABSTRACT

An apparatus is disclosed for obtaining ellipsometric measurements from a sample. A probe beam is focused onto the sample to create a spread of angles of incidence. The beam is passed through a quarter waveplate retarder and a polarizer. The reflected beam is measured by a detector. In one preferred embodiment, the detector includes eight radially arranged segments, each segment generating an output which represents an integration of multiple angle of incidence. A processor manipulates the output from the various segments to derive ellipsometric information.

PRIORITY

[0001] This application claims priority to provisional applicationserial No 60/315,514, filed Aug. 28, 2001, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] There is ongoing interest in expanding and improving themeasurement of semiconductor wafers. A number of optical metrology toolshave been developed for non-destructively evaluating the characteristicsof thin films formed on semiconductors during the fabrication process.More recently, optical metrology systems have been proposed foranalyzing the geometry of small periodic structures (criticaldimensions) on semiconductors.

[0003] Typical optical tools include reflectometry (both singlewavelength and spectroscopic) and ellipsometry (again, both singlewavelength and spectroscopic.) In some metrology tools, these varioustechniques are combined. See for example U.S. Pat. Nos. 6,278,519 and5,608,526, the disclosures of which are incorporated herein byreference.

[0004] Other metrology tools have been developed which rely onmeasurements at multiple angles of incidence (both single wavelength andspectroscopic). One class of such systems have been commercialized bythe Assignee herein are capable of deriving information about multipleangles of incidence simultaneously. In these systems, a strong lens(high numerical aperture) is used to focus a probe beam of light ontothe sample in a manner to create a spread of angles of incidence. Anarray detector is used to measure the reflected rays of the probe beamas a function of the position within the probe beam. The position of therays within the probe beam corresponds to specific angles of incidenceon the sample. Theses systems are disclosed in U.S. Pat. Nos. 4,99,014and 5,042,951, incorporated herein by reference. U.S. Pat. No. 4,99,014related to reflectometry while U.S. Pat. No. 5,042,951 relates toellipsometry. (See also, U.S. Pat. No. 5,166,752, also incorporatedherein by reference).

[0005] In a variant on this system, U.S. Pat. No. 5,181,080(incorporated by reference), discloses a system in which a quad-celldetector (FIG. 1) is used to measure the reflected probe beam. Eachquadrant 1-4 of the detector measures an integration of all of theangles of incidence falling on the sample. By subtracting the sums ofopposite quadrants, ellipsometric information can be obtained. Asdescribed in the latter patent, the information derived from theanalysis corresponds to the ellipsometric parameter delta 6 which isvery sensitive to the thickness of very thin films on a sample.

[0006] The concepts of the latter patents were expanded to providespectroscopic measurements as described in U.S. Pat. No. 5,412,473, alsoincorporated herein by reference. In this patent, the system wasmodified to include a white light source. In one approach, a colorfilter wheel was used to sequentially obtain multiple wavelengthinformation. In another approach, a filter in the form of a rectangularaperture was used to select a portion of the reflected beam. Thisportion was then angularly dispersed onto an array with each rowproviding different wavelength information and each column containingthe various angle of incidence information.

[0007] U.S. Pat. No. 5,569,411, also incorporated by reference,disclosed a preferred approach for obtaining spectroscopic informationfor an integrated multiple angle of incidence system of the typedescribed in U.S. Pat. No. 5,181,080 discussed above. In this approach,a filter was provided that transmitted light along one axis and blockedlight along an orthogonal axis. The transmitted light was angularlydispersed and measured to provide spectroscopic information along oneaxis of the probe beam. The filter was then rotated by ninety degrees toobtain measurements along the remaining axis. Various modifications ofthis approach were discussed, including splitting the beam and using twoidentical filters disposed orthogonal to each other to obtain bothmeasurements simultaneously. (See also “Characterization of titaniumnitride (TiN) films on various substrates using spectrophotometry, beamprofile reflectometry, beam profile ellipsometry and spectroscopic beamprofile ellipsometry,” Leng, et. al. Thin Solid Films, Volume 313-314,1998, pages 309 to 313.)

[0008] The integrated multiple angle ellipsometric measurement systemdescribed in U.S. Pat. No. 5,181,080, cited above has been successfullycommercialized and is incorporated into the Opti-Product sold by theAssignee herein. The technology is marketed under the trademark BeamProfile Ellipsometry. (See U.S. Pat. No. 6,278,519 cited above.) Asdescribed in the '080 patent, the four segments of the quad celldetector can be summed to provide information about the total reflectedpower of the probe beam. In addition, the sum of the output of thequadrants along one axis can be subtracted from the sum of the outputsof the remaining two quadrants to provide a result which is correspondsto the ellipsometric parameter δ.

[0009] This arrangement provides valuable information that can be usedto determine the thickness of thin films. However, the limitedinformation from this type of detection cannot typically be used toderive both of the ellipsometric parameters, Ψ and δ. U.S. Pat. No.5,586,411, discloses that it would be possible to derive suchinformation if one of polarizers were rotated and multiple measurementstaken. As noted therein at column 12, line 48, if enough measurementsare taken, a Fourier analysis can be performed on the data allowing theparameters of Ψ and δ to be extracted.

[0010] When designing commercial inspection systems, it is oftendesirable to minimize the number of moving parts. For example, movingparts often create particulates that can contaminate the wafer. To theextent parts must be moved, the motion systems must have high precision.Further, movements of parts that are specifically designed to modifyoptical properties, such as retarders or polarizers can effect how thesystem transmits and detects light.

[0011] Therefore, it is an object of the present invention to enhancethe operation of an integrated, simultaneous multiple angleellipsometric system without the drawbacks of the prior approaches. Inparticular, the subject invention is intended to permit the derivationof additional ellipsometric information, including both δ and Ψ. In oneclass of embodiments, this additional information is derived in a systemwith an improved detector arrangement without the need for moving parts.In another class of embodiments, the rotating element is limited to thedetector which does not effect the polarization or retardation of thelight.

SUMMARY OF THE INVENTION

[0012] In a first embodiment, a narrowband light source such as a laseris used to generate a probe beam. The polarized beam is focused onto thesample in a manner to create a large spread of angles of incidence. Theprobe beam light is passed through a quarter waveplate for retarding thephase of one of the polarization states of the beam with respect to theother polarization state of the beam. A polarizer is provided forcreating interference between the two polarization states in thereflected probe beam.

[0013] In accordance with the subject invention, a detector is providedwith eight segments radially arranged around a center axis. In oneembodiment, eight pie shaped sections are provided in a configurationwhich is essentially a quad cell with each quadrant further divided inhalf. In another embodiment, the eight segments are arranged in anannular ring.

[0014] In either case, the output of the segments lying substantiallyalong one radial axis is subtracted from the output of the segmentslying substantially along an orthogonal radial axis. In order to gainadditional information, the output of the sectors lying along a thirdradial axis located midway between the first two orthogonal axes (i.e.at 45 degrees) is subtracted from the sectors lying along a fourthradial axis, perpendicular to the third axis. This extra informationobtained corresponds to an orientation essentially shifted by 45 degreesfrom the first measurements. When all the measurements are combined, thesample may be more accurately evaluated. This extra information can besupplied to conventional fitting algorithms to evaluate characteristicsincluding thin film thickness, index of refraction and extinctioncoefficient. In addition, geometrical parameters of structures formed onsemiconductors such as line width, spacing, and side wall angle andshape can also be evaluated. These calculations can be made using themeasurements directly. Alternatively, the measurements can be used toderive the ellipsometric parameters Ψ and δ which are then used toevaluate the sample.

[0015] The use of an eight segment detector allows the extra measurementto be obtained simply by summing and subtracting outputs of the sectorsin the processor. Other arrangements can be used to provide equivalentresults. For example, the reflected beam could be split into two partsand the two quadrant detectors use, one offset by 45 degrees from theother. Alternatively, a single rotatable quadrant detector could beused. After the first measurement is made, the detector could be rotatedby 45 degrees and a second measurement could be made. In a preferredembodiment, the output of all the segments is summed to provide ameasure of the full power of the reflected beam.

[0016] It may also be possible to rotate the either the polarizer or theretarder to achieve a similar result.

[0017] The concept can also be extended to the use of a two dimensionaldetector array. Using a processor, the elements on the array can becomputationally mapped to the eight segment and the analysis can be madeas described above. This approach can be particularly useful formeasurement of critical dimensions, where the orientation of the samplestructure and orientation of the probing radiation is significant andpossibly difficult to control.

[0018] The subject invention can also be extended to spectroscopicmeasurements. In this case, a white light source would typically be usedto generate a polychromatic probe beam. The probe beam could be passedthrough a color filter or monochrometer which sequentially transmitsnarrow bands of wavelengths. The filter or monochrometer wouldpreferably be located before the sample.

[0019] If simultaneous multiple wavelength information is desired, it isbelieved that the approach described in U.S. Pat. No. 5,596,411, whichincluded a rotating quadrant filter, grating and array detector would bemore suitable than the approach described herein.

[0020] Further objects of the subject invention can be understood withreference to the following detailed description, taken in conjunctionwith the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a schematic diagram of a quad-cell detector used inprior art measurements.

[0022]FIG. 2 is a schematic diagram of the optical lay-out of anapparatus for performing the method of the subject invention.

[0023]FIG. 3 is a schematic diagram of an eight segment detector for usewith the subject invention.

[0024]FIG. 4 is a schematic diagram of a detector having eight detectorarms aligned along four axes which can be used to implement the subjectinvention.

[0025]FIG. 5 is a schematic diagram of a detector having segments in theform of an annular ring which can be used to implement the subjectinvention.

[0026]FIGS. 6A and 6B are schematic diagrams of a two dimensionaldetector array which can be used to implement the subject invention.

[0027]FIGS. 7A and 7B are a schematic diagrams of a rotatable quadrantdetector which can be used to implement the subject invention.

[0028]FIG. 8 is a schematic diagram of a pair of quadrant detectorswhich can be used to implement the subject invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Turning to FIG. 2, an apparatus 10 is illustrated for performingthe method of the subject invention. The apparatus lay out for thisembodiment is essentially the same as that described in U.S. Pat. No.5,181,080, except that the detector is configured with eight segments(FIG. 3) rather than four segments as in the prior art (FIG. 2). Theapparatus is designed to evaluate characteristics at the surface of asample 14, such as thin film layers 12 and/or structural features suchas critical dimensions.

[0030] In this embodiment, apparatus 10 includes a light source 20 forgenerating a probe beam 22 of radiation. One suitable light source is asolid state laser diode which emits a linearly polarized beam having astable, known and relatively narrow bandwidth. Probe beam 22 is turnedtowards the sample 14 with a 50/50 beam splitter 24. The probe beam isfocused onto the surface of the sample with a lens 26. In the preferredembodiment, lens 26 is defined by a spherical, microscope objective witha high numerical aperture on the order of 0.90 NA. The high numericalaperture functions to create a large spread of angles of incidence withrespect to the sample surface. The spot size is on the order of twentymicrons or less and is preferably five microns or less in diameter.

[0031] In should be noted that in this illustrated embodiment, the beamis directed substantially normal to the surface of the sample prior tobeing focused by lens 26. This configuration helps minimize the spotsize on the sample. It is within the scope of the subject invention todirect the beam at a non-normal angle of incidence to the sample asshown in U.S. Pat. No. 5,166,752. Although using an off-axis beamincreases the spot size on the sample, high angles of incidence can becreated with a lower numerical aperture lens.

[0032] Turning back to FIG. 1, a fraction of the probe beam power alsopasses through splitter 24 and falls on an incident power detector 30.As is well known to those skilled in the art, incident power detector 30is provided to monitor fluctuations in the output power of the probebeam light source. As discussed in U.S. Pat. No. 5,181,080, the incidentpower detector can be modified to minimize measurement errors whicharise due to asymmetries of the beam.

[0033] Light reflected from the surface of the sample passes up throughsplitter 24 towards photodetector 40. Prior to reaching detector 40, thebeam 22 is passed through a quarter-wave plate 42 for retarding thephase of one of the polarization states of the beam by 90 degrees. Itshould be noted that the quarter-wave plate could be located in the beampath prior to the probe beam striking the sample so that the systemwould operate with circularly polarized light. The latter approach mighthave some advantages in reducing the aberrations created by lens 26. Inaddition, while a phase retardation of 90 degrees will maximize thedesired signal, other intermediate levels of retardation would bepossible.

[0034] The beam is then passed through a linear polarizer 44 whichfunctions to cause the two polarization states of the beam to interferewith each other. In order to maximize the desired signal, the axis ofthe polarizer should be oriented at an angle of 45 degrees with respectto the fast and slow axes of the quarter-wave plate 42.

[0035] In accordance with the subject invention, detector 40 isconfigured to generate independent signals from regions along two pairsof mutually orthogonal axes. In this first embodiment, this goal isachieved by using a photodetector having eight pie shaped segments. Asillustrated in FIG. 3, the detector surface includes eight, radiallydisposed segments 1-8. Each segment will generate an output signalproportional to the magnitude of the power of probe beam striking thatquadrant. This signal represents an integration of the intensities ofall the rays having different angles of incidence with respect to thesample surface. While this integration approach results in the loss ofsome information content as compared to an analysis of individual rays,the composite approach does provide significantly greater sensitivitythrough enhanced signal to noise performance.

[0036] The probe beam 22 should be centered on the detector 40 so thateach segment intercepts an equal portion of the probe beam. The probebeam should underfill the detector.

[0037] The output of the segments is supplied to the processor 50 forevaluation. As in the prior art, the outputs of all the segments can besummed to provide a measure of the full power of the reflected beam. Asdiscussed below, this full power measurement can be used as an input toa regression analysis to determine the characteristics of the sample.

[0038] In accordance with the invention herein, the processor can alsogenerate measurements which allow additional ellipsometric informationto be derived as compared to the prior art approach. This difference canbest be understood by comparing the two approaches.

[0039] In general, one measures the total reflectivity of the sample inaccordance with the following equation:

R=½(|r _(p)|²⁺ |r _(s)|²)

[0040] The sine of the ellipsometric phase shift δ is determined by theequation:

tan Ψ e ^(iδ) =|r _(p) /r _(s) |e _(iδ)

[0041] With the prior art detector of FIG. 1, the information is derivedas follows:

Σ==1+2+3+4

Δ=(1+3)−(2+4)

R=Σ/Σ_(o) and sin δ=λ/2(Δ/|r _(p) r _(s)|)

[0042] where Σ₀ is the measured sum signal from a known referencematerial.

[0043] The complete determination of the polarization state requires ameasurement that gives tan ψ and that requires an additionalmodification to the detector as shown in FIG. 3. As noted above, the sumis formed from the outputs of the eight segments as follows:

Σ=1+2+3+4+5+6+7+8

[0044] Then as previously:

R=Σ/Σ_(o) and with

Δ₁=(1+2+5+6)−(3+4+7+8)

Δ₂=(2+3+6+7)−(4+5+8+1)

[0045] one has

Δ₁/Σ=4/λ(tan ψ/tan²ψ+1)sinδand

Δ₂/Σ=−2/λ(tan²ψ−1/tan²ψ+1)

[0046] As can be seen, the information from the eight segments can beused to derive both Ψ and δ. In practice, those quantities may not beneeded. In fact, it is often preferable to use the measurements moredirectly in the evaluation of sample parameters. Thus, the inventionshould not considered limited to determining both Ψ and δ, but rather isan approach which provides an additional measurement for analyzing thesample.

[0047] Although the measurements made by the subject apparatus could beused by themselves to characterize a sample, those measurements can alsobe combined with other measurement obtained from additional opticalmetrology devices in manner discussed in U.S. Pat. No. 6,278,519. Asystem with multiple inspection technologies generates a number ofindependent measurements which are then combined in a regressionanalysis to determine sample parameters.

[0048] Combining data from multiple devices is a procedure quite wellknown and need not be described in detail. In brief, a mathematicalmodel is defined which describes the structure under test. A best guessof sample parameters is assigned to the model and the optical responseis calculated. The calculated optical response is compared to themeasured optical response. Any deviations between the calculated opticalresponse and the measured optical response are used to vary the initialstarting parameter guesses and the process is repeated in an iterativefashion until satisfactory convergence is reached. (See, for example,the Leng, article cited above.) As noted above, with such an analyticalapproach it is not necessary to actually calculate Ψ and δ. Rather theinputs from Δ₁ and Δ₂ calculations as set forth above (as well as thefull power measurement) are used as inputs actual measurements to thefitting algorithm. Of course, if desired, the Δ₁ and Δ₂ calculations canalso be used to calculate Ψ and δ if desired.

[0049] These types of analyses are suitable for both thin films andphysical structures formed on the sample. It is also possible to use adatabase or library type approach where a set of the optical responsesof a parameterized sample are calculated in advance and stored. Themeasured response is compared to the stored responses to determinesample parameters. (See, for example, US Published Applications2002/0038186 and 2002/0035455). The subject invention is not intended tobe limited either by the type of sample being measured, nor the specificalgorithms used to analyze the data. As will be discussed below, thereare a number of alternative detector configuration which can be used togenerate the information of interest. One common thread is thatmeasurements are taken along a first pair of orthogonal axes and along asecond pair of orthogonal axes, with the second pair being perpendicularto the first pair.

[0050]FIG. 4 illustrates an alternative configuration for such adetector 440 that satisfies these criteria. Each segment 1-8 is a lineardetector arranged in a star-shaped configuration that corresponds to thepie shaped segments of FIG. 3. The analysis of the measurementsdiscussed above with respect to FIG. 3 would be identical to that ofFIG. 4.

[0051] Those skilled in the art will also appreciate that segments 5 to8 for either the FIG. 3 or FIG. 4 embodiment are complimentary tosegments 1 to 4 so that a detector with only segments 1 to 4 might beused. However, adding the outputs of segments 5 to 8 to the outputs ofsegments 1 to 4, respectively, makes the detector insensitive to smallshifts in the probe beam spot positioning and is therefore the preferredapproach.

[0052] The detector configurations of FIGS. 3 and 4 will produce ameasurement that represents an average over all the incident angles. Toexamine a narrower range of angles of incidence, a detector 540 as shownin FIG. 5 with eight segments arranged in an annulus might be used. Sucha detector might be of interest where the probe beam is directedsubstantially normal to the sample before focusing as shown in FIG. 2.In such a case, the radially outermost rays of the probe beam have thehighest angles of incidence. The annular ring configuration of FIG. 5would capture only those higher angle of incidence rays, which, in thecase of isotropic samples, often carry the most information.

[0053] The subject invention could also be implemented using a detector640 comprised of a two dimensional array of detector elements or pixelsas shown in FIG. 6A. Such a detector could be defined by a CCD array.The pixels in the array could be mapped into eight segments tocorrespond to the detectors shown in FIGS. 3 to 5. The mapping of pixelsto the detector segments of FIG. 3 is shown in FIG. 6B. The processorwould select the output from the appropriate pixels to calculate Δ₁ andΔ₂ and derive the ellipsometric information.

[0054] With a sufficiently dense array almost any configuration ofangles of incidence is possible. This is especially significant formeasuring physical structures (CD) where the xy orientation of thesample structure and the orientation of probing radiation is relevantand significant. With a system such as described above it is possible(with minimal moving mechanisms) to probe the CD structure from allangles of incidence, planes of incidence and polarizations relative tothe orientation of the CD structure.

[0055] If it is desired to extend this concept to measure multiplewavelengths, the laser light source 20 could be a white light sourcethat would generate a polychromatic probe beam. A wavelength selectivefilter 60 (shown in phantom line in FIG. 1) would then be placedsomewhere in the light path between the light source and the detector.The filter could take the form of simple band pass (color) filters whichare selectively moved into the path of the beam. Alternatively, amonochrometer could be used to sequentially select narrow wavelengthregions. Of course a tunable laser or multiple lasers with differentwavelengths could also be used.

[0056]FIGS. 7 and 8 illustrate two further embodiments wherein detectorswith only four quadrants can be used to obtain the measurements requiredherein. FIG. 7a illustrates a quadrant detector 740 positioned to take afirst set of measurements. Segment 1 would generate an output equivalentto segments 1 and 2 of the detector of FIG. 3. Similarly, segment 2 ofthe detector of FIG. 7a would generate an output equivalent to segments3 and 4 of the detector of FIG. 3, segment 3 of the detector of FIG. 7awould generate an output equivalent to segments 5 and 6 of the detectorof FIG. 3 and segment 4 of the detector of FIG. 7a would generate anoutput equivalent to segments 7 and 8 of the detector of FIG. 3. Thismeasurement would permit the calculation of Δ₁.

[0057] Once this measurement is made, the quadrant detector could berotated 45 degrees to a position shown in FIG. 7b. In this orientation,segment 1 of the detector of FIG. 7b would generate an output equivalentto segments 2 and 3 of the detector of FIG. 3. Similarly, segment 2 ofthe detector of FIG. 7b would generate an output equivalent to segments4 and 5 of the detector of FIG. 3, segment 3 of the detector of FIG. 7bwould generate an output equivalent to segments 6 and 7 of the detectorof FIG. 3 and segment 4 of the detector of FIG. 7b would generate anoutput equivalent to segments 8 and 1 of the detector of FIG. 3. Thismeasurement would permit a calculation of Δ₂.

[0058] The detector configuration of FIG. 7 might be desirable tosimplify and minimize the cost of the detector. However, the trade offwould be that the measurement would require two steps and the rotationof an element. Rather than rotating the detector, similar results mightbe achieved if either the polarizer or analyzer or both were rotated toprovide independent measurements.

[0059]FIG. 8 illustrates a configuration conceptually similar to FIG. 7but which would allow both measurements to be taken at once. Morespecifically, after the probe beam is reflected from the sample andpasses the polarizer and waveplate, it can be divided by beam splitter802. Two separate quadrant detectors 840 and 842 are each located in oneof the two beam paths. The quadrants of detector 840 are offset from thequadrants of detector 842 by 45 degrees. The output of the quadrants ofdetector 840 can be used calculate Δ₁ and the output of the quadrants ofdetector 842 can be used to calculate Δ₂. As an alternative, one couldplace the beam splitter in the path of the reflected probe but beforethe quarter waveplate and polarizer. In this alternative, it would benecessary to place a quarter waveplate and a polarizer in each path. Inthis arrangement, it would also be possible to orient both detectors atthe same azimuthal angle, but have different positions for either thewaveplate or the polarizer in one path as compared to the other path.

[0060] While the subject invention has been described with reference toa preferred embodiment, various changes and modifications could be madetherein, by one skilled in the art, without varying from the scope andspirit of the subject invention as defined by the appended claims.

I claim:
 1. An apparatus for evaluating the characteristics of samplecomprising: a light source for generating a probe beam; a focusingelement for focusing the beam onto the surface of the sample in mannerto create a spread of angles of incidence with varying azimuthal angles;a retarder located in the path of the beam; a polarizer located in thepath of the reflected beam; a detector having eight radially disposedsegments oriented to measure the reflected beam, each segment generatingseparate output signals; and a processor for evaluating thecharacteristics of the sample based on the output signals.
 2. Anapparatus as recited in claim 1, wherein said light source is a narrowband laser.
 3. An apparatus as recited in claim 1 or 2, wherein saidsegments are arranged in a pie shaped configuration.
 4. An apparatus asrecited in claim 1 or 2, wherein said segments are arranged in anannular ring.
 5. An apparatus as recited in claim 1 or 2, wherein theprocessor operates to calculate the difference between the sum of theoutputs of segments disposed along a first pair of orthogonal axes andthe difference between the sum of the outputs of segments disposed alonga second pair of axes, perpendicular to the first pair of axes, toevaluate the sample.
 6. An apparatus for evaluating the characteristicsof sample comprising: a light source for generating a probe beam; afocusing element for focusing the beam onto the surface of the sample inmanner to create a spread of angles of incidence with varying azimuthalangles; a retarder located in the path of the beam; a polarizer locatedin the path of the reflected beam; detector means for measuring thepower of the reflected probe beam along a first pair of orthogonal axesand a second pair of orthogonal axes disposed at a 45 degree azimuthalangle with respect to the first pair of axes and generating outputsignals corresponding to each of the four axes; and a processor forevaluating the characteristics of the sample based on the outputsignals.
 7. An apparatus as recited in claim 6, wherein said lightsource is a narrow band laser.
 8. An apparatus as recited in claim 6 or7, wherein the processor operates to calculate the difference betweenthe sum of the outputs along axes of the first pair and the differencebetween the sum of the outputs along the axes of the second pair toevaluate the sample.
 9. An apparatus as recited in claim 6 or 7, whereinthe detector means is defined by a detector having eight radiallydisposed segments oriented to measure the reflected beam, each segmentgenerating separate output signals.
 10. An apparatus as recited in claim9, wherein said segments are arranged in a pie shaped configuration. 11.An apparatus as recited in claim 9, wherein said segments are arrangedin an annular ring.
 12. An apparatus as recited in claim 6 or 7, whereinthe detector means is defined by a detector with four radially arrangedquadrants and wherein said detector can be rotated about the propagationaxis of the beam to obtain measurements at different azimuthal angles.13. An apparatus as recited in claim 6 or 7, wherein the detector meansis defined by a pair of detectors, each detector having four radiallyarranged quadrants, said detector means further including a beamsplitter for directing the beam along separate paths to each of the twodetectors and wherein the azimuthal position of the first detector isoffset from the second detector to obtain two different measurements.14. An apparatus as recited in claim 6 or 7, wherein the detector meansis defined by a two dimensional array of photodetector elements andwherein the processor uses information obtained from elements lyingwithin eight different radially disposed segments to evaluate thecharacteristics of the sample based on the output signals.
 15. Anapparatus as recited in claim 6 or 7, wherein the detector means isdefined by a two dimensional array of photodetector elements and whereinthe processor correlates the individual outputs from the detectingelements to specific angles of incidence and a plurality azimuthalpositions, said azimuthal positions including two orthogonal axes and atleast two more azimuthal positions intermediate the orthogonal axes. 16.An apparatus as recited in claim 6 or 7, wherein the detector means isdefined by a two dimensional array of photodetector elements and whereinthe processor correlates the individual outputs from the detectingelements along a first pair of orthogonal axes and along second pair oforthogonal axes, perpendicular to the first pair in order to evaluatethe sample.
 17. An apparatus as recited in claim 16, wherein theprocessor operates to calculate the difference between the sum of theoutputs of segments disposed along said first pair of orthogonal axesand the difference between the sum of the outputs of segments disposedalong said second pair of axes.
 18. An apparatus as recited in claim 1or 6, wherein the light source generates a polychromatic probe beam andfurther including a selectable color filter located in the path of thebeam and wherein the output signals vary as a function of wavelength.19. A method of evaluating the characteristics of sample comprising thesteps of: focusing a probe beam onto the surface of the sample in mannerto create a spread of angles of incidence with varying azimuthal angles;retarding the phase of one polarization state of the probe beam withrespect to he phase of the other polarization state; interfering the twopolarization states in the probe beam after the probe beam has beenreflected from the surface of the sample; measuring the power of thereflected probe beam along a first pair of orthogonal axes and a secondpair of orthogonal axes, perpendicular to the first pair and generatingoutput signals corresponding to each of the four axes; and evaluatingthe characteristics of the sample based on the output signals.